Is it possible? I don't know much about batteries and the like, other than Duracell cost a bastid fortune on Christmas day from the local garage.
However I've ridden a couple of e bikes, one being that hai bike enduro pro beast and whilst flying up hill wondered if the then good part of any climb (the downhill on the other side) could return the energy back into the batteries. Too much tech to fit on to a small chassis such as a bike?
You have to have something to spin the motor. In bikes current form that would mean a direct drive so no freewheeling or another motor in the hub.
Not an expert mind you. Just an educated guess.
That "Copenhagen Wheel" works using regen, I think...
Most modern brushless hub motors - Heinzmann DirectDrive, BionX, Go SwissDrive etc - can regenerate. I'm not sure it adds an awful lot to range in normal use, do you really want something slowing you down on the fun downhills?
because Kinetic Energy is 1/2 x M x V^2 and bikes are light and only go slowly, there is actually very little to be gained from regen on a bike in most cases! The total round trip efficiency is in the order of 65% typically, so you'll only be able to get back less than you put in.
A bike (especially a mountain bike) is broadly about drag, ie over coming friction, and that energy is lost and cannot be recovered.